#36210 streams: Include the OS error when AutoFile I/O fails

• Include the underlying OS error message when file stream I/O operations fail
• Help users and callers diagnose exact failure causes like missing permissions or full disks

Captures errno immediately after failed fread/fwrite calls in AutoFile and BufferedFile and appends the OS error message via SysErrorString() to thrown std::ios_base::failure exceptions. Adds unit test coverage in streams_tests to verify error string propagation across POSIX and Windows platforms.

Problem: AutoFile and BufferedFile throw fixed exception strings on real I/O failures, discarding errno and preventing callers from diagnosing why an I/O operation failed (such as out-of-space or permission errors).

P3 · diagnostics

• P3 because it preserves errno to give meaningful failure details across the node
• Provides a durable diagnostic improvement to shared file utilities without addressing critical bugs

Improves diagnostics on stream I/O failures across the node by preserving errno. While not fixing a critical bug or security issue, providing meaningful error reasons rather than generic failure messages is a clear, durable improvement to shared utility code.

Membership: Changes core stream primitives in src/streams.h and src/streams.cpp.

Factors: security/stability 0, bug 1, performance 0, user value 1, leverage 1

• Ready for review
• Small and self-contained with passing tests and no blockers

The PR is small, self-contained, tests pass, and there are no blockers or unaddressed reviewer requests.

Author status: active

Resolved concerns:

• Alpine musl libc CI failure resolved by closing the file descriptor to trigger a genuine kernel-level EBADF rather than relying on libc-specific direction checks.

• No reviews or concept feedback yet

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The PR has had no reviewer comments since opening.

Review verdicts (DrahtBot): 0

Enables:

• Better error reporting for callers switching to AutoFile, such as wallet dump in #35492

98 lines under test/bench/ci.

• src/test/streams_tests.cpp +98/-0
• src/streams.cpp +9/-4
• src/streams.h +3/-1

This PR updates AutoFile and BufferedFile stream implementations to capture errno on failing fread and fwrite calls, appending the formatted system error string to thrown std::ios_base::failure exceptions. This ensures error messages surfaced to logs and callers distinguish underlying OS causes such as out-of-space, bad descriptors, or permission issues. Comprehensive unit tests are added to verify error propagation across platforms. The pull request is clean, has passing CI, and currently awaits initial reviewer attention.